1. Field of the Invention
The present invention relates to apparatus and methods for calibrating and compensating Delta Sigma ADCs and DACs by correlating noise signals.
2. Description of the Prior Art
FIG. 1 (prior art) shows a stereo analog to digital converter (ADC) comprising two delta sigma ADC""s 104, 110 followed by decimation and filtering blocks 106, 112. In typical stereo systems the input signals are a right signal 102 and a left signal 108. It is known in the art to attempt to compensate for some imperfection in one or both of the delta sigma DAC""s by inserting a correction block 114. In conventional systems the correction done by block 114 is measured by inputting a carefully controlled input test signal and measuring the output signal. If the input signal is unstable or the environment is noisy, the correction is inaccurate.
Delta sigma converters are subject to intersymbol interference (ISI), mismatched levels (in the case of multilevel quantization) mismatched components, and various other hardware based imperfections that limit their accuracy and vary from converter to converter. In general, it is much cheaper to correct errors in digital hardware than in analog. Past efforts to measure and correct errors of this sort have focused on measuring signal levels given a carefully controlled input test signal. A need remain in the art for apparatus and methods for calibrating and compensating Delta Sigma ADC""s and DAC""s by correlating noise signals.
The present invention correlates noise from a delta sigma converter with noise from the same converter passed through a nonlinear block whose purpose is to isolate the imperfection being measured. Once the imperfection is measured the results may be used to correct the output of the delta sigma modulator, to accept or reject chips or for other purposes.
Apparatus according to the present invention for measuring circuit imperfections in an analog to digital converter (ADC) having a delta sigma modulator comprises a first decimator connected to the output of the modulator, a nonlinear test element connected to the output of the modulator, for applying a nonlinear function to the output of the modulator, a second decimator matched to the first decimator and connected to the output of the nonlinear element, a correlator connected to the output of the first decimator and the output of the second decimator, and means for applying a test signal to the ADC. The correlator performs a correlation between the output of the first decimator and the output of the second decimator in the presence of the test signal and generates a circuit error signal based upon the correlation. The apparatus may also include means for applying a correction factor, based upon the circuit error signal, to the output of the ADC as the ADC operates in real time with a realtime input signal.
The correction factor may be applied by trimming a circuit element in the ADC, by placing a correction block connected between the modulator and the first decimation filter for correcting the output of the ADC according to the circuit error signal.
This apparatus may be used to measure intersymbol interference. In this case, the nonlinear element outputs a predetermined signal only when the output of the modulator, in the presence of the test signal, supplies a predetermined pattern. Or, in the case where the delta sigma modulator is multilevel and includes a digital to analog converter (DAC) in the feedback path, the apparatus can be used to measure mismatch in components associated with each level in the DAC. A swap control element in the DAC selectively swaps elements in the DAC associated with each level in order to test all of the levels.
Apparatus according to the present invention may also be used to for measure circuit imperfections in a stereo (or more channel) analog to digital converter (ADC) including at least two analog delta sigma modulators connected to inputs of the ADC. In this case the apparatus comprises a first decimator connected to the output of the first modulator, a nonlinear test element connected to the output of the first modulator, for applying a nonlinear function to the output of the first modulator, a second decimator matched to the first decimator and connected to the output of the nonlinear element, a correlator connected to the output of the first decimator and the output of the second decimator, and means for applying a test signal to the ADC. Again, the correlator performs a correlation between the output of the first decimator and the output of the second decimator in the presence of the test signal and generates a circuit error signal based upon the correlation. In the case where a correction factor, based upon the circuit error signal, is applied to an output of the ADC in real time, the nonlinear element is disabled (for example by removing its connection to the second decimator) and the second decimator is connected to the output of the second modulator.
Apparatus according to the present invention for measuring circuit imperfections in a MASH ADC including a set of delta sigma modulators comprises a first set of filter elements, one of the first set connected to the output of each modulator, means for combining outputs of the first set of filter elements to form a first filtered output, a plurality of nonlinear test elements, one of the test elements connected to the output of each modulator, for applying a nonlinear function to the output of that modulator, a second set of filter elements matched to the first set of filter elements, one of the second set connected to the output of each nonlinear element, means for combining outputs of the second set of filter elements to form a second filtered output, a correlator connected to the output of the first means for combining outputs and the second means for combining outputs, and means for applying a test signal to the MASH. The correlator performs a correlation between the first filtered output and the second filtered output in the presence of the test signal, and generates a circuit error signal based upon the correlation. The apparatus may include means for applying a correction factor, based upon the circuit error signal, to the output of the MASH in the presence of a real time input signal to the input of the MASH.
In a stereo (or more channel) MASH ADC including a two sets of delta sigma modulators, when the correction factor is applied, the nonlinear elements and disabled and each of the second set of filter elements is connected to one of the second set of delta sigma modulators.
Apparatus according to the present invention for measuring circuit imperfections in a delta sigma digital to analog converter (DAC) including a digital delta sigma modulator comprises a first simple DAC connected to the output of the modulator, a nonlinear test element connected to the output of the modulator, for applying a nonlinear function to the output of the modulator, a second simple DAC matched to the first simple DAC and connected to the output of the nonlinear element, a correlator connected to the output of the first simple DAC and the output of the second simple DAC, and means for applying a test signal to the DAC. The correlator performs a correlation between the output of the first simple DAC and the output of the second simple DAC in the presence of the test signal and generates a circuit error signal based upon the correlation. A correction factor based upon the circuit error signal may be applied to the output of the delta sigma DAC in real time.
The correction factor may be applied by trimming a circuit element the first simple DAC, or by adjusting the feedback path of the delta sigma modulator. The simple DACs might be PWMs or 1-bit DACs.
This apparatus may be used to measure intersymbol interference, by having the nonlinear element output a predetermined signal only when the output of the delta sigma DAC in the presence of the test signal supplies a predetermined pattern.
Similar apparatus may be used for measuring circuit imperfections in a stereo (or more channel) delta sigma digital to analog converter (DAC) including at least two digital delta sigma modulators connected to inputs of the delta sigma DAC. In the case where a correction factor, based upon the circuit error signal, is applied to an output of the DAC in real time, the nonlinear element is disabled (for example by removing its connection to the second simple DAC) and the second simple DAC is connected to the output of the second modulator.
A method according to the present invention of measuring circuit imperfections in a delta sigma digital to analog converter (DAC) including a digital delta sigma modulator includes the steps of applying a nonlinear function to the output of the modulator, performing a correlation between the output of the DAC and the output of the DAC after the nonlinear function is applied, and generating a circuit error signal based upon the correlation. The step of correcting the output of the DAC according to the circuit error signal may be added.
In this method, the test signal may be a signal out of the frequency of interest of the DAC, and the steps of generating a circuit error signal and correcting the output of the DAC can then be repeated as the DAC operates.
Or, the step of generating the circuit error signal may be performed once, or at selected times, and the step of correcting the output of the DAC according to the circuit error signal occurs as the DAC operates.
A similar method may be used to measure circuit imperfections in a delta sigma analog to digital converter (ADC) including an analog delta sigma modulator. The steps are applying a nonlinear function to the output of the modulator, performing a correlation between the output of the modulator and the output of the modulator after the nonlinear function is applied, and generating a circuit error signal based upon the correlation. Again the step of correcting the output of the ADC according to the circuit error signal may be added. If the test signal is a signal out of the frequency of interest of the ADC, the steps of generating a circuit error signal and correcting the output of the ADC can be repeated as the ADC operates. Or, the step of generating the circuit error signal can be performed once, or at selected times, and the step of correcting the output of the ADC according to the circuit error signal occurs as the ADC operates.